Generic WAL Records

Chapter 60. Generic WAL Records

Although all built-in WAL-logged modules have their own types of WAL
records, there is also a generic WAL record type, which describes changes
to pages in a generic way. This is useful for extensions that provide
custom access methods, because they cannot register their own WAL redo
routines.

The API for constructing generic WAL records is defined in
access/generic_xlog.h and implemented
in access/transam/generic_xlog.c.

To perform a WAL-logged data update using the generic WAL record
facility, follow these steps:

state = GenericXLogStart(relation) — start
construction of a generic WAL record for the given relation.

page = GenericXLogRegisterBuffer(state, buffer, flags)
— register a buffer to be modified within the current generic WAL
record. This function returns a pointer to a temporary copy of the
buffer's page, where modifications should be made. (Do not modify the
buffer's contents directly.) The third argument is a bitmask of flags
applicable to the operation. Currently the only such flag is
GENERIC_XLOG_FULL_IMAGE, which indicates that a full-page
image rather than a delta update should be included in the WAL record.
Typically this flag would be set if the page is new or has been
rewritten completely.
GenericXLogRegisterBuffer can be repeated if the
WAL-logged action needs to modify multiple pages.

Apply modifications to the page images obtained in the previous step.

GenericXLogFinish(state) — apply the changes to
the buffers and emit the generic WAL record.

WAL record construction can be canceled between any of the above steps by
calling GenericXLogAbort(state). This will discard all
changes to the page image copies.

Please note the following points when using the generic WAL record
facility:

No direct modifications of buffers are allowed! All modifications must
be done in copies acquired from GenericXLogRegisterBuffer().
In other words, code that makes generic WAL records should never call
BufferGetPage() for itself. However, it remains the
caller's responsibility to pin/unpin and lock/unlock the buffers at
appropriate times. Exclusive lock must be held on each target buffer
from before GenericXLogRegisterBuffer() until after
GenericXLogFinish().

Registrations of buffers (step 2) and modifications of page images
(step 3) can be mixed freely, i.e., both steps may be repeated in any
sequence. Keep in mind that buffers should be registered in the same
order in which locks are to be obtained on them during replay.

The maximum number of buffers that can be registered for a generic WAL
record is MAX_GENERIC_XLOG_PAGES. An error will be thrown
if this limit is exceeded.

Generic WAL assumes that the pages to be modified have standard
layout, and in particular that there is no useful data between
pd_lower and pd_upper.

Since you are modifying copies of buffer
pages, GenericXLogStart() does not start a critical
section. Thus, you can safely do memory allocation, error throwing,
etc. between GenericXLogStart() and
GenericXLogFinish(). The only actual critical section is
present inside GenericXLogFinish(). There is no need to
worry about calling GenericXLogAbort() during an error
exit, either.

GenericXLogFinish() takes care of marking buffers dirty
and setting their LSNs. You do not need to do this explicitly.

For unlogged relations, everything works the same except that no
actual WAL record is emitted. Thus, you typically do not need to do
any explicit checks for unlogged relations.

The generic WAL redo function will acquire exclusive locks to buffers
in the same order as they were registered. After redoing all changes,
the locks will be released in the same order.

If GENERIC_XLOG_FULL_IMAGE is not specified for a
registered buffer, the generic WAL record contains a delta between
the old and the new page images. This delta is based on byte-by-byte
comparison. This is not very compact for the case of moving data
within a page, and might be improved in the future.